1. Field of the Invention
This invention relates generally to biomedical thermometers, and more particularly, to improvements in infrared thermometers for measuring body temperature.
2. Description of Related Art
Infrared thermometers have become useful in the medical community and permit the rapid measurement of a patient's temperature. Various approaches have been developed to make the infrared thermometer more accurate. One prior approach involves alternately sensing the radiation from an inner reference area and the target through the same optical path in order to obtain a more accurate temperature measurement. Another type of approach involves the use of a chopper unit to calibrate the thermometer. In the case of infrared thermometers which use a thermopile as the infrared detector, approaches specific to thermopile accuracy have been developed.
Typically, thermopile detectors which are used to measure infrared radiation produce an output voltage which increases as a non-linear function of the difference between the temperature of the heat sensing area in the detector and the temperature of the cold junctions of the detector. Thermopile detectors are commonly used for the measurement of extremes of temperatures, such as in detecting the temperature of a furnace, or in PG,4 detecting fire. In the case of the application of the thermopile detector to infrared thermometry for the measurement of human body temperature, the temperature range is relatively narrow when compared to typical thermopile applications. Because of this narrow temperature range, techniques to improve accuracy of the thermopile detector have been under consideration.
It has been found that changes in the ambient temperature affect the temperature of the cold junctions which in turn can affect the accuracy of the thermopile output unless the temperature of the cold junctions is considered appropriately. One method used to improve the accuracy of patient temperature measurement with the thermopile detector is to maintain the cold junctions at a constant temperature by heating them to a predetermined temperature which is above the ambient temperature, or cooling them to a temperature below ambient. However, this method requires a heat source, power for the heat source and techniques for the precise regulation of the temperature of the heat source.
An ideal thermopile detector generates an output voltage that is related to the target's temperature and the temperature of the cold junctions by the following formula which was derived from the fundamental Stefan-Boltzmann law: EQU V.sub.d =M.multidot.(T.sub.BB.sup.4 -T.sub.CJ.sup.4)
where V.sub.d is the thermopile detector output voltage, T.sub.BB is the target temperature (in this case a blackbody), T.sub.CJ is the temperature of the cold junctions, and M is a constant.
In actual practice however, there exist many factors which alter this ideal relationship. One factor is that our knowledge of the temperature of the cold junctions, T.sub.CJ, is typically subject to error. In most cases, a contact temperature sensor for the cold junctions is removed from the cold junctions somewhat thus making its temperature measurement inaccurate to some extent. Additionally, M is not truly constant and often depends upon the ambient temperature or the age of the detector or other factors.
A prior method for determining the blackbody target temperature T.sub.BB from the output signal V.sub.d of a thermopile detector and the temperature of the thermopile detector cold junctions T.sub.CJ was by the following equation involving a polynomial series: EQU T.sub.BB =T.sub.CJ +A.sub.1 .multidot.V.sub.d +A.sub.2 .multidot.V.sub.d.sup.2 +A.sub.3 .multidot.V.sub.d.sup.3 +A.sub.4 .multidot.V.sub.d.sup.4 + . . . +
By manipulating the equation, it can be seen that the polynomial series in the thermopile detector output voltage equals T.sub.BB -T.sub.CJ. However, it has been observed that this voltage is actually more dependent on the T.sub.CJ term than this equation indicates. It has been observed that even if T.sub.BB changes the same amount that T.sub.CJ changes, in actual practice V.sub.d will not remain the same. By not accounting for this occurrence, accuracy will be adversely affected.
Thus it would be desirable to provide a method and a system for enhancing the accuracy of the output signal of a detector by considering the cold junction temperature or temperature of the reference temperature area of the detector more fully.
The tympanic membrane is generally considered by the medical community to be superior to other sites for taking a patient's temperature. The same blood source which supplies the hypothalamus, the organ which regulates body temperature, also supplies the tympanic membrane and thus the temperature of the tympanic membrane is closely correlated to the core temperature. However, the tympanic membrane is not directly accessible; it does not lie in a straight path from the ear opening. Therefore, obtaining infrared energy from only the tympanic membrane and not from surrounding tissues, such as the ear canal, is difficult when merely inserting an instrument into the ear opening. Yet it is desirable to be able to measure a patient's temperature in this manner; quickly and non-intrusively. Whether the temperature detector is a thermopile or another type of detector, some adjustment of its output may be necessary to more accurately indicate the temperature of the tympanic membrane. Additionally, it would be desirable to determine the temperature of a particular part of a patient's anatomy, such as the core, the temperature of which may be accurately correlated with the temperature of an infrared target part of anatomy, such as the tympanic membrane, by measuring the temperature of the target and adjusting that temperature as necessary to indicate the temperature of the other part of the anatomy.
It would be desirable to provide a method and system which provide accurate temperature measurements of a patient's anatomy over a wide temperature range without requiring the application of a heat source or a cold source to the detector.
For continued accuracy of the instrument, it would also be desirable to provide a method and system allowing re-calibration of the biomedical thermometer periodically in the field. The present invention addresses these needs.